1. Field of the Invention
The present invention relates to a current monitoring circuit for a chargeable/dischargeable secondary battery, and relates, more particularly, to a current monitoring circuit for monitoring a charging/discharging current of a secondary battery incorporated in a portable information terminal.
2. Description of the Related Art
In recent years, along with standardization of power source management for portable information terminal, there has been required a high-precision detection of the remaining capacity of a secondary battery. Therefore, it is necessary to monitor with high precision a charging/discharging current that flows to a secondary battery.
In a conventional portable information terminal incorporating a secondary battery, a voltage generated by a charging/discharging current flowing through a current sensing resistor connected in series with the battery or an ON resistance of an FET (Field Effect Transistor) provided for interrupting a charging/discharging route, is amplified, and this voltage is analog/digital (A/D) converted. The A/D converted signal is calibrated by a microprocessor (MPU) or the like.
In the monitoring of a charging/discharging current, when only an amplifier circuit for monitoring a large charging/discharging current is used, it is not possible to monitor a small charging/discharging current with high precision because of limits in the input range and the resolution of the A/D converter. On the other hand, when an attempt is made to achieve high-precision monitoring of a charging/discharging current by using amplifier circuits of different amplification degrees for monitoring a large current charging, a large current discharging, a small current charging/discharging current respectively, there is a problem of an increase in the circuit scale.
In other words, according to the conventional current monitoring circuit, when an increase in the circuit scale is restricted, it is not possible to monitor a current with high precision. As a result, there occurs an error in the battery remaining capacity valve. On the other hand, when a plurality of amplifier circuits of different amplification degrees are used, there is a problem of increase in the circuit scale, which results in a high-cost current monitoring circuit.
In order to solve the above problems, it is an object of the present invention to provide a high-precision and low-cost charging/discharging current monitoring circuit for a secondary battery.
According to the present invention, there is provided a current monitoring circuit for a secondary battery, comprising: a current detector detecting a current during a charging and a discharging of a secondary battery, and outputting a voltage signal according to a detected current; an amplifier having an amplification degree that can be changed and amplifying the voltage signal output from the current detector at a suitable amplification degree obtained by changing the amplification degree; an A/D converter for converting an output of the amplifier into a digital signal; and a numerical-value converter converting the digital signal output from the A/D converter according to an amplification degree of the amplifier, to thereby output a digital value indicating a current during the charging and the discharging of the secondary battery.